As is known, the portion of the foot that has the greatest perspiration effect is the sole.
Therefore, the region of the shoe where moisture produced by perspiration can accumulate most is the interface between the sole of the foot and the sole of the shoe.
Here, the sweat that is produced saturates the air with moisture and mostly condenses, stagnating on the footbed.
Only a marginal fraction of the moisture produced by perspiration diffuses to the sides of the upper and exits from them if they are vapor-permeable.
This effect of stagnation of perspiration in the plantar region is particularly conspicuous in shoes that have a rubber sole; in such cases vapor permeation through the sole is in fact prevented by its total waterproofness.
As is known, the stagnation of sweat in the plantar region produces in the user of the shoe a feeling of discomfort and constitutes a preferential location for the growth of bacterial cultures, which notoriously cause bad odors.
The need to obviate the stagnation of moisture produced by perspiration at the plantar region of shoes is therefore a commonly felt need.
A first attempt to meet this need consists of the solution proposed in Italian patent no. 1232798.
The teaching contained in said patent consists in dividing the rubber sole into two layers, the lower of which has through micro-holes, and in interposing between them a semipermeable membrane that is joined perimetrically to the two layers, in order to avoid infiltrations of water and thus obtain a sole that is impermeable to water in the liquid state and is permeable to water vapor.
For the sake of simplicity, hereinafter an element with the property of being impermeable to water in the liquid state and permeable to water vapor is indicated as waterproof and vapor-permeable.
The semipermeable membranes that the inventor of Italian patent no. 1232798 teaches to use are for example of the type described in U.S. Pat. Nos. 4,187,390 and 4,194,041 in the name of W. L. Gore or U.S. Pat. No. 6,228,477 in the name of BHA Technologies.
These membranes are provided by means of thin films of expanded polytetrafluoroethylene, e-PTFE, with thicknesses that generally vary from 15 to 70 microns, and are waterproof and vapor-permeable.
Their microstructure is characterized by the presence of dense areas, known as nodes, which are interconnected by elongated filaments, known as fibrils.
These semipermeable membranes, initially conceived for the military sector, have been developed and used in the field of clothing and shoes to avoid the accumulation of water vapor caused by perspiration in items of clothing and provide shoes with uppers with waterproof and vapor-permeable linings.
Since the market of the clothing and shoes sectors has always required soft and comfortable items, in the described applications there is a strong need to make sure that the membrane, understood as a functional layer, does not compromise these characteristics.
This requirement has developed into an actual technical prejudice, which has entailed the use of membranes provided in low thicknesses in order to be laminated with support and/or aesthetic finishing materials, such as fabric or leather, so as to obtain finished laminates that have enhanced characteristics of flexibility, ease of bending, softness, surface slipperiness, compressibility and extensibility and low weight per unit surface.
However, the films that provide these membranes have poor characteristics of mechanical strength, indeed due to their low thickness.
Indeed, it should be noted that the value of resistance of the laminate derives mainly from the characteristics of the layer of fabric or of the support with which the membrane is coupled.
In particular, available films of polymeric material, used to provide said membranes as mentioned, have thicknesses generally from 15 to 70 microns, which give them limited penetration resistance, i.e., less than 5 N.
The expression “penetration resistance” is used to reference the characteristic defined by a measurement performed according to the method presented in the ISO 20344-2004 standard in chapter 5.8.2, “Determination of the penetration resistance of the sole” related to safety shoes.
This limited mechanical resistance to penetration has led the inventor of the Italian patent no. 1232798 to prevent contact of the membrane with foreign objects by limiting the diameter of the holes of the sole which the membrane faces.
However, this solution has proved to limit drastically the area of the sole assigned to vapor permeation; moreover, the holes can become blocked.
The teaching contained in European patent no. 858,270 by the same Applicant proposes a solution that is aimed at overcoming these drawbacks but is still not entirely free from aspects that can be improved.
This patent illustrates a shoe with a sole made of elastomer which is perforated and comprises a midsole that comprises a waterproof and vapor-permeable membrane superimposed on a lower protective layer, preferably made of felt, which is treated so as to be water-repellent.
Since the protective layer is not made of waterproof material, it is not possible to perform a direct seal of the midsole with the sole, but a perimetric waterproof element is used which provides a sealing bridge between the waterproof and vapor-permeable membrane and the sole.
As mentioned, this invention, while allowing effective protection of the membrane against the penetration of external objects, has some aspects that can be improved.
In particular, the connection of the protective layer to the membrane, in order to allow an effective protection thereof, must occur by way of their intimate adhesion.
For this purpose, adhesives and glues are used which, while not covering all of the surface of the membrane, limit the vapor-permeable portion of the part that they affect.
Moreover, the protective layer itself, despite being vapor-permeable, is an obstacle to the passage of water vapor from the inside to the outside of the shoe through the midsole.
This drawback increases when, during use of the shoe, the protective is layer progressively loses its characteristics of water-repellence and therefore tends to become impregnated with water and/or mud or other types of dirt, absorbed through the holes of the tread, compromising the vapor permeability of the sole.
Another drawback consists in that during use the cyclic flexing and traction to which the midsole is subjected during walking tends to cause progressive wear and tearing of the membrane, thus causing a loss of waterproofness to the sole.
In order to obviate this drawback, European patent no. 858,270 teaches to provide soles that have small holes that are sufficiently spaced to not compromise their structural strength.
In this manner, the sole supports the midsole so as to contrast the tearing of the membrane.
However, the portion of sole that can be crossed by water vapor is limited indeed by the rarity and narrowness of the holes with which it is provided.
Finally, the presence of multiple layers inside the sole limits its ability to flex, making it rigid, to the full disadvantage of user comfort.
A further solution, aimed at proposing a compromise between the need to contrast the tearing of the membrane and the need to allow effective disposal of water vapor produced by perspiration through the sole, is disclosed in Italian patent no. 1.334.928 by the same Applicant.
This patent discloses a sole that has a structure that comprises a supporting layer which, at least in one macroportion, is made of mesh, felt or other diffusely perforated material.
A membrane made of material that is waterproof and permeable to water vapor is associated in an upward region with the supporting layer so as to cover at least the macroportion thereof.
Moreover, a sole made of polymeric material, with at least one macrohole that passes through at the macroportion, is joined so as to form a seal to the membrane and to the supporting layer at the perimeter of the macroportion.
This patent contains the teaching of structuring the membrane by enhancing it with tear-resistant layers, made for example of nylon mesh intimately associated with the waterproof and vapor-permeable membrane, protective layers and also high-strength layers made for example of Kevlar fabric.
The presence of the macrohole allows to define a large surface of the membrane that is adapted for heat exchange and for the exchange of water vapor with the outside of the shoe; simultaneously, the loss of structural stiffness of the sole is compensated, to contrast the tearing of the waterproof and vapor-permeable membrane, by the supporting layer.
The advantage provided by having a large portion of sole free for vapor permeation is reduced, however, by the obstacle to the passage of water vapor that is constituted by the presence of the several layers that compose the supporting layer.
A further solution, aimed at obtaining an effective disposal of water vapor produced by perspiration through the sole, is disclosed in European patent EP 1185183 by the same Applicant.
This patent discloses the method for manufacturing a vapor-permeable sole that consists of a first step in which a unitary upper assembly is formed which comprises a membrane made of waterproof and vapor-permeable material and a protective element made of a material that is resistant to hydrolysis, water-repellent, breathable or perforated.
The upper is fixed directly to the membrane in a lower region.
A second step consists in mutually fixing the upper assembly thus obtained and a sole made of perforated elastomer by means of a joint with a perimetric seal.
The protective element is arranged below the membrane, in a region comprised between the upper part of the sole and its inner part, which is adjacent to the ground contact surface.
The text of the patent contains the teaching of directly associating by gluing the insole or the upper with a waterproof and vapor-permeable membrane, which is integrated with supporting layers, made for example of mesh of synthetic material, and with protective layers, made for example of non-woven fabric or needle fabric or Kevlar fabric.
The advantage offered by providing the membrane on the upper assembly, however, is reduced by the obstacle to the passage of water vapor that is constituted by the presence of the several layers that compose the insole or upper, the supporting layer and the protective layer and all the layers of adhesive interposed between.
Currently known vapor-permeable shoes, and particularly those with a vapor-permeable sole, therefore suffer several drawbacks, and in particular are                complicated, due to the presence of many superimposed layers,        rigid, since the materials that constitute the several layers limit the flexibility of the polymeric material of the sole,        generally perceived as not being comfortable, when the inevitable hollows caused by the different stratification of the underlying elements are felt through the insole,        scarcely vapor-permeable, due to the overlap of multiple layers, and to the presence of the adhesive needed to join them.        